Polybutylene terephthalate (PBT) is a widely-used, high performance engineering resin that can be processed to make parts for automotive, electrical, and industrial applications. A commercial process for manufacturing PBT typically includes a series of reactors for performing transesterification and polycondensation reactions.
The transesterification step in the production of PBT generally involves reacting dimethyl terephthalate (DMT) with excess 1,4-butanediol (BDO) at high temperature (i.e., 190° C.) in the presence of a catalyst to form bishydroxylbutyl terephthalate (BDO ester), as well as other compounds, for example, as shown in Reaction 1 as follows:
Reaction 1 is an equilibrium reaction and is driven forward by removal of the methanol produced.
The polycondensation step in the production of PBT involves the use of heat and vacuum to polymerize the transesterification reaction product. The transesterification product, for example, the BDO ester that is formed from Reaction 1, polymerizes in the presence of a catalyst and at high temperature (i.e. 240° C.) to form PBT, for example, as shown in Reaction 2 as follows:
The polycondensation reaction is an equilibrium reaction and is driven forward by removal of the BDO produced.
Undesirable side reactions occur in the transesterification/polycondensation process for manufacturing PBT. Certain significant side reactions form tetrahydrofuran (THF), for example, as shown in exemplary Reactions 3 and 4 as follows:
The formation of THF is undesirable because BDO reacts to form THF, as seen in Reaction 3, thereby reducing the amount of BDO that is converted to PBT. Reaction 3 occurs in the early stages of polymerization when the concentration of BDO is high. During the later stages of polymerization, high processing temperatures promote production of THF from BDO-terminated polymer end groups, leading to a higher concentration of acid-terminated PBT chain ends, as seen in Reaction 4. Reaction 4 also demonstrates a link between THF formation and the concentration of acid-terminated chain ends of the PBT produced, since an acid-terminated chain end is produced for each molecule of THF formed via this pathway.
Acid groups in the final PBT polymer are undesirable, because they may adversely affect polymer properties, for example, hydrolytic and melt stability. High acid content also leads to corrosion problems when the PBT comes into contact with metal during injection molding or other polymer processing. Further disadvantages are described herein below with respect to the depolymerization of high acid PBT to form macrocyclic polyester oligomer.
A typical industrial process for making PBT includes unit operations for handling raw materials and products, as well as a series of reactors for performing transesterification, prepolycondensation, polycondensation, and solid state polymerization. Transesterification may be performed in a single- or multi-stage reactor. DMT and BDO are mixed and heated as they are fed into the reactor. The reaction mixture boils as methanol and THF are produced. The methanol vapor is condensed and recovered in a condenser.
A prepolycondensation step is then typically performed using one or more reactors operating at high temperature and low pressure (i.e. vacuum). In a prepolycondensation step, BDO produced during polymerization (i.e. Reaction 2) is removed using heat and vacuum. The BDO and final traces of methanol are recovered using condensers. A pump forwards molten polymer through a mixer, where stabilizers and additives may be introduced.
Polycondensation is the final stage of melt polymerization. Polycondensation requires a special reactor, for example, a rotating disc reactor such as a Vickers-Zimmer reactor, in order to facilitate the removal of BDO that drives polymerization. The polycondensation reactor is designed to remove BDO by providing a large amount of continuously-renewed surface area. BDO typically is removed in order to build the molecular weight of the PBT product.
A solid state polymerization step may be performed after melt polymerization in order to increase the molecular weight of the PBT. Solid state polymerization involves pelletizing the polymer produced in the polycondensation step and heating the pellets in a fixed bed until crystallization occurs. The polymer is then maintained at high temperature (i.e., 200° C.) while a stream of inert gas passes through the fixed bed to carry away the BDO formed during polymerization. The solid state polymerization step may take up to 18 hours or more.
Processes for the commercial manufacture of PBT are expensive. Capital costs are high, due to the special reactors needed for transesterification, prepolycondensation, polycondensation, and solid state polymerization. For example, the transesterification reactors must be designed to minimize sublimation of reactant DMT, the prepolycondensation reactors must be designed to operate at high temperatures and high vacuum (low pressure), and the polycondensation reactors must be designed to provide continuously renewed surface area for BDO removal during polymerization. Processing costs are also high, due to various factors including the cost of maintaining the high temperatures and low pressures of the reactors, the cost of the catalyst required, and inefficient conversion of reactants due to the loss of BDO as THF, for example.
Methods have been proposed for reducing THF formation in the manufacture of PBT. For example, U.S. Pat. No. 5,516,879 by Yuo et al. and U.S. Pat. No. 5,519,108 by Yuo et al. describe the use of a multi-component catalyst system to accelerate polycondensation. These patents suggest that the use of an alkali metal phosphate as a co-catalyst along with tetrabutyl titanate or tetraisopropyl titanate reduces formation of THF in the preparation of PBT from DMT. Another method for reducing THF formation is described in the article entitled, “Effect of salts on the formation of THF in preparation of PBT by TPA process,” by Chang and Tsai, J. Appl. Polym. Sci., 45 (2), pp. 371-373 (1992). This article proposes the use of potassium and sodium salts to lower the amount of THF formed in PBT production. The article describes application of the technique in conjunction with the direct reaction of BDO and terephthalic acid (TPA) to produce PBT. Special equipment is necessary for commercial applications involving direct esterification with TPA because TPA sublimes and cannot be easily purified by distillation.
Despite industry improvements, a significant percentage (i.e. 3 wt. %) of reactant BDO is typically lost as THF in present commercial PBT manufacturing processes.
Furthermore, commercially manufactured PBT has a high acid content. Various methods have been proposed for reducing the acid concentration of PBT. For example, diepoxides have been used to reduce the acid content of PBT from 44 mmol/kg to 10 mmol/kg. See Gooijer et al., “Carboxylic acid end group modification of poly(butylene terephthalate) in supercritical fluids,” Polymer, 44 (8), pp. 2201-2211 (2003). Another method for reducing the acid concentration of PBT is described in U.S. Pat. No. 5,854,377 by Braune. This patent describes the use of an alkali metal or alkaline earth metal compound to reduce the concentration of acid groups in PBT. Another proposed method of reducing acid groups in PBT is to add diol just before final polycondensation to directly react with the terminal carboxylic acid groups. However, the addition of a diol may decrease the polymerization reaction rate or even reverse the reaction, thereby producing lower molecular weight polymer.
Despite industry improvements, commercially manufactured PBT typically has an acid concentration greater than about 35 meq/kg, although some specially-manufactured PBT has an acid content as low as 7 meq/kg. Various high-grade PBT's that are commercially available include Valox® 315 manufactured by GE Plastics of Pittsfield, Mass. (38 meq acid/kg), Ultradur® B6550 manufactured by BASF Corporation of Wyandotte, Mich. (19 meq acid/kg), and Celanex® 2001 manufactured by Ticona Engineering Polymers of Shelby, N.C. (7 meq/kg). Valox® 315 and Ultradur® B6550 are melt-polymerized PBT's, while Celanex® 2001 is a solid state-polymerized PBT.
PBT may be depolymerized to form macrocyclic polyester oligomers (MPO's), including, for example, the cyclic form of poly(1,4-butylene terephthalate) (cPBT). MPO's have unique properties that make them attractive as matrix-forming resins for engineering thermoplastic composites. MPO's lend valuable characteristics to polymerized products, for example, high strength, high gloss, and solvent resistance. Furthermore, because certain MPO's melt and polymerize at temperatures well below the melting point of the resulting polymer, polymerization and crystallization can occur virtually isothermally upon melting of the MPO in the presence of an appropriate catalyst. The time and expense required to thermally cycle a tool is favorably reduced, because demolding can take place immediately following polymerization, without first cooling the mold.
Various methods for preparing MPO by depolymerizing polyesters have been described. See, e.g., co-owned U.S. Pat. No. 5,039,783 by Brunelle et al., U.S. Pat. No. 5,231,161 by Brunelle et al., U.S. Pat. No. 5,407,984 by Brunelle et al., U.S. Pat. No. 5,668,186 by Brunelle et al., U.S. Pat. No. 6,525,164, by Faler, and U.S. Pat. No. 6,787,632 by Phelps et al., the texts of which are all incorporated by reference herein in their entirety.
Depolymerization of commercially-available PBT into cPBT typically requires a high catalyst concentration. For example, the above-mentioned U.S. Pat. No. 5,668,186 by Brunelle et al. describes depolymerization of PBT using from about 1.0 to about 5.0 mole percent of a titanium catalyst based on total moles of polyester monomer units. The depolymerization reaction typically progresses relatively slowly and produces undesired byproducts, including hydroxybutylester linear oligomers, which are separated from the product stream. These byproducts are typically gellular in nature, and are physically difficult to remove.
Furthermore, residual acid typically is removed from the cPBT product stream, by, for example, costly treatment using alumina-packed columns. The more acid that is present, the more costly the treatment. Finally, commercially-available PBT suitable for depolymerization into cPBT is costly, due in part to the expense involved in its production.
Thus, for effective manufacture of cPBT, there is a need for less costly starting materials. There is also a need for PBT that has lower acid end group concentrations. Furthermore, there is a need for a faster, more efficient, less costly method for depolymerizing PBT into its cyclic form.